Bring salt and water to a boil in a heavy saucepan over high heat. Whisk in grits, and cook, whisking constantly, 45 seconds. Scrape bottom and sides of the pot.
Return to a boil; cover and reduce heat to medium-low. Cook 20 to 25 minutes or until tender. (For a looser consistency, whisk in 2 to 4 Tbsp. water halfway through cooking.)
Stir in butter until melted, and serve immediately.